Today, satellite systems exist for supporting telecommunications with, and/or providing geolocation information to, user units. Satellite based telecommunications systems, such as Odyssey (as proposed by the assignee of the present application), utilize a constellation of satellites to relay communications signals between user terminals and earth or base stations. The user terminals are assigned to earth stations. The earth stations direct calls to and from the assigned user terminals. The user terminals and associated earth stations communicate along preassigned communications channels having a preassigned bandwidth (subband) centered about a carrier frequency.
Satellite based navigation systems, such as the global positioning satellite (GPS) system, include a constellation of satellites which transmit navigation signals to the user units. Each satellite emits a unique navigation signal along a preassigned navigation channel. User units obtain navigation information from multiple navigation signals and, based thereon, calculate the terminal's position relative to the earth. One GPS technique is explained in an article entitled "GPS Signal Structure and Performance Characteristics", by J. J. Spilker, Jr., Global Positioning System, which is incorporated by reference in its entirety.
The navigation and communications signals are transmitted over separate, mutually exclusive channels specifically designated to carry corresponding signals. Consequently earth stations, satellites and user terminals must be designed to support separate communications and navigation channels, thereby undesirably complicating the overall system. Existing telecommunications systems have not been able to merge communications and geolocation signals/data onto a single RF signal.
As the popularity of cellular telecommunications increases, more and more demands are being placed upon the capacity of telecommunications satellite systems. These demands effectively monopolize the available communications subbands. Satellite systems have attempted to increase the overall capacity of the available frequency subbands by utilizing a variety of user-access or "spread spectrum-based" techniques to increase the user-density within a given frequency subband. These user-access techniques include frequency division multiple access (FDMA coding), time division multiple access (TDMA coding), and code division multiple access (CDMA coding). In addition, hybrid techniques have been proposed using a combination of TDMA, FDMA and CDMA codes. Depending upon the coding technique, each user terminal when assigned to a corresponding channel, is given a unique TDMA/FDMA/CDMA code and/or transmission timing/frequency. The user terminals transmit and receive all communications signals at the assigned carrier channel, code and transmission timing/frequency.
As the coding techniques increase the user density, acceptable tolerances decrease between adjacent user channels before co-channel interference results. Therefore, the above-mentioned coding techniques require the communications link between a user terminal and an earth station to be adjusted or tuned continually. Such adjustments are necessary to ensure that the user terminal continues to transmit within its assigned channel as the user terminal and/or coverage satellites move relative to one another.
User terminals and earth stations transmit telecommunications signals as discrete packets or frames of information. Several of the above-mentioned coding techniques require that the communications link be maintained "synchronous" between the earth station and the user terminal. A "synchronous" communications link requires that each frame of data be received (at a user terminal or an earth station) at an instant in time simultaneous with receipt of frames transmitted from other terminals and/or earth stations. The frames must also be received in the assigned subband centered about an assigned carrier frequency. Thus, synchronization and subband alignment are determined with respect to the receiver.
However, synchronization and subband alignment are continuously effected 1) by variations in range between the satellites and user terminals or earth stations (e.g., propagation delay) and 2) by changes in the relative range velocity between the satellite and the user terminals or earth stations (e.g., Doppler effect).
Propagation delay variation arises as user units and/or coverage satellites move since the distance or range changes between a user terminal and its associated earth station. Consequently, the propagation time of a frame between an earth station and user terminal changes. To compensate for variations in the propagation time, the transmitter (e.g., earth station or user terminal) retards or advances the initiating time at which data frames are transmitted. Thus, transmitters located near an intended receiver retard the transmission initiating time, while transmitters located far from the receiver advance the transmission initiating time.
Doppler shift variation arises as satellites move relative to earth stations and user terminals since the resultant carrier frequency perceived at a receiver changes. For instance, when a satellite moves toward a user unit, the perceived carrier frequency at the satellite (and at the subsequent earth station) of a communications signal is greater than the actual carrier frequency at which the user terminal transmitted the communications signal. Stated another way, the perceived carrier frequency at the receiver is greater than it would otherwise be if the satellite and transmitter remained at rest with respect to one another. Similarly, when the satellite moves away from the transmitter, the receiver perceives a carrier frequency which is lower than the carrier frequency which would be otherwise perceived if the satellite were not moving relative to the transmitter. This phenomena is referred to as the "Doppler effect". Transmitters (e.g., user terminals and earth stations) adjust the carrier frequency, at which communications signals are transmitted, to account for positive and negative Doppler shifts. By adjusting the frequency and timing, a transmitter ensures that communications signals transmitted therefrom remain within the assigned channel and arrive synchronous with signals from other transmitters.
The timing and frequency may be controlled in several manners. For instance, the earth station may transmit timing and frequency update information to each user terminal.
However, conventional telecommunication systems limit the use of the telecommunications channels to transmission of telecommunications information. Consequently, in conventional telecommunications systems separate channels must be used to provide other functions such as navigation and the like.
Moreover, systems which provide GPS-type navigation information require an unduly large number of satellites. To calculate geolocation, GPS user terminals require simultaneous line-of-sight navigation links with multiple satellites. Consequently, the GPS system necessitates a constellation of satellites which ensure that multiple satellites are simultaneously viewable at each position covered by the system. This multi-satellite coverage technique unduly increases the number of satellites.
A need remains within the industry for an improved geolocation method and apparatus for use with satellite based telecommunications. It is an object of the present invention to meet this need.